P_s inversion in three homologous series of ferroelectric 5-(2-fluoro-alkyloxy)-2-(4-n-alkylphenyl) pyrimidines

Abstract

We synthesized members of four homologous series of the general formula

Most of the compounds exhibit smectic phases with negative P_S, at higher temperatures, changing the sign to give positive P_S, at lower temperatures. The inversion temperatures and P_S, at a reduced temperature show alternating behavior. The tilt angles measured by X-rays as well as by electrooptical investigation do not possess any anomaly at T_PS, and in several cases are practically constant. We discuss the molecular arrangemeit in C and C* phases. In order to explain the P_S behavior we use the molecular statistical theory developed by Osipov et al. In this theory Ps is determined by the chiral dipole, the steric dipole and the polarizability properties of the molecules; the interplay of these properties characterizes the binding site. By opposite signs of dipolar and quadrupolar coupling of the molecules and the temperature dependence of the steric dipoles, the P_s inversion can be explained.